Co-owned U.S. Pat. No. 7,295,731 B2 describes a method for using femtosecond laser radiation for the creation of optical waveguiding devices inside standard optical fibers. The femtosecond laser is focused inside the optical fiber and the intense light generated in the focal region produces non-linear absorption in the glass fiber and creates permanent refractive index changes. The different refractive index regions can be used to produce waveguide vias for light to travel, thus functionalizing the fiber for applications beyond their original intent. For example, these waveguides can be made to interact with the fiber core and allow light to be coupled from the core to the fiber cladding. This technique can be used to form applications such as local light tapping for power monitoring, cladding waveguides for sensing, periodic structures such as Bragg grating for light filters, and others.
Photonic crystal fibers have been established for years and they confine and guide light, not by having a core with a positive index difference to its surroundings, but instead by creating a light band gap, with subsequent confinement, around the center of the fiber by means of strategically positioned hollow (usually air filled) holes. The band gap structure guides the light in the center of the fiber, typically a solid core with a material that is the same as the material composing the rest of the solid portions of the fiber (typically fused silica/quartz glass). However, in certain configurations, the core itself may also be formed by a hollow hole and still guide the light in its center. Other types of fibers such as suspended core fibers also utilize hollow structure to form a cladding surrounding a solid core. In this case, the hollow areas are typically larger than the core itself and are connected by thin solid bridges that hold the structure together.
The hollow holes in all these types of fibers, with their very low refractive index when compared to glass fiber, are fundamental to the creation of a photonic band gap and to the working principle of this class of light waveguides. However these structures also present a challenge for the fabrication of waveguides with femtosecond lasers. Since there is no material to be modified inside those holes, the methods that form the current state of the art are not adequate and the introduction of other non-obvious elements to the fabrication procedures are required in order to have a means to achieve the same potential for the local creation of devices as is currently possible in standard optical fibers with femtosecond laser writing.